1. Field of the Invention
The present invention relates to a method of measuring an amount of organic material adsorbed to a surface of a semiconductor substrate, which adversely influences the electrical characteristics of the substrate, and which is a factor causing various defects in the product.
2. Description of the Related Art
For the purpose of maintaining a high yield of semiconductors and high reliability of the produced device, it is essential to improve the total cleaning technique. This tendency will be more prominent in the future process technique for downsizing devices and for processing wafers having an larger diameter. In the total cleaning technique for the semiconductor production, it is particularly important to reduce the contamination due to impurities adsorbed to the surface of a semiconductor substrate. If an impurity is adsorbed to a semiconductor, troubles are likely to occur in the steps following the total cleaning. The etching rate and the film being formed may be adversely affected. The electrical characteristics of an insulation film, including the voltage-proof property, may deteriorate.
Not only metal impurities which have been regarded as adversely influencing characteristics of a substrate, but also organic impurities adsorbed to the surface of the substrate. Therefore, the amount of organic material impurities as well as that of metal impurities must be measured in order to determine the amount of impurities to be removed for reducing the influence by contamination.
A conventional method of measuring organic material adsorbed to the surface of a semiconductor substrate, is disclosed in the article "Impurity Removal Method for Semiconductor Surface" in "KOKAI GIHOU (Technical Report Publication 46489.times.006-1". As can be seen in FIG. 1 of this article, a semiconductor substrate 303, to which organic material is adsorbed, is immersed into tens of milliliters of extrapure water 301 filled in a quartz container 302, and is left to stand for a certain period of time. Thus, the organic material is removed from the semiconductor substrate 303 and dissolved into the extrapure water 301. The solution now containing the organic material is sampled and analyzed by use of a total organic carbon meter (to be called "TOC meter" hereinafter).
As described above, in the conventional method, the semiconductor substrate is immersed into tens of milliliters of extrapure water filled in a quartz container, to remove the organic material from the semiconductor substrate. Obviously, extrapure water is required in a great amount to remove a very small amount of the organic material. Further, the detection sensitivity of the TOC meter is limited to 1 to 2 ppb. Therefore, the smallest amount of organic material (adsorbed to the surface of a semiconductor substrate,) which the TOC meter can detect (hereinafter called "detection limit amount") is 10.sup.13 atoms/cm.sup.2 in terms of the number of carbons (C) per unit area. More specifically, if the solution is used in an amount of 50 ml, and the TOC meter has a detection limit of 1 ppb, the detection limit amount will be calculated as follows: ##EQU1##
The value obtained by the equation (1) can be converted to the number of carbons (C) by the equation (2): ##EQU2##
The value obtained by the equation (2) can be converted to the number per unit area of semiconductor substrate, by the equation (3). Since the total surface area (including those of the front and rear surfaces) of a 6-inch substrate is about 350 cm.sup.2, the number per unit area can be obtained by the following equation (3): ##EQU3##
Also, in the conventional method described above, organic material is removed from a semiconductor substrate, simply by immersing the semiconductor substrate into extrapure water. Therefore, if the organic material is insoluble to extrapure water, the material cannot not be completely removed. As a result, the efficiency of removal is low, and the sensitivity of the measurement is inevitably low.
A method of measuring a metal impurity adsorbed to the surface of a semiconductor substrate is described, for example, in Jap. Pat. Appln. KOKAI Publication No. 2-28533. In this method, flameless atomic absorption spectrometry is applied for analyzing a metal impurity, and a droplet containing a metal impurity is atomized by heat in, for example, a high-purity graphite carbon furnace (not shown). In the flameless atomic absorption spectrometry, light is emitted from a hollow cathode lamp (not shown) to atomic vapor dissociated due to high temperature in the furnace, and the intensity of the resonant ray absorbed in the atomic vapor is detected. The amount of metal in a droplet can be calculated from the detected intensity. In this metal impurity measuring method, the metal impurity removed from a semiconductor substrate is collected by an acid droplet having a volume of about 0.1 ml. Therefore, the detection limit amount can be in the order of 10.sup.9 to 10.sup.10 atoms/cm.sup.2.
As can be understood from the above, the first-mentioned conventional method can measure an amount of organic material, but with a sensitivity 3 to 4 orders of magnitude lower than the second-mentioned conventional method measures an amount of metal impurity.